Add armor to the body without long weight paint
Enter edit mode, select one vertex of the armor, shift select a vertex of the other piece of the armor, Ctrl L to select all linked vertices, in the vertex group tab click assign with a weight of 1.0.





Another workflow: select a vertex from the body, Ctrl L to select all linked, H to hide the body, switch to weight paint mode and do all the paints you need (Face selection mode must be on, with all the vertices selected), Alt H to make the body reappear.